In this section we consider how one party can authenticate another party when the two are communicating over a network. We focus here on authenticating a "live" party, at the point in time when communication is actually occurring. We will see that this is a subtly different problem from proving that a message received at some point in the past (e.g., that may have been archived) did indeed come from that claimed sender. This latter problem is referred to as the digital signature problem, which we explore in section 7.4.
When performing authentication over the network, the communicating parties can not rely on biometric information, such as a visual appearance or a voiceprint. Indeed, we will see in our later case studies that it is often network elements such as routers and client/server processes that must authenticate each other. Here, authentication must be done solely on the basis of messages and data exchanged as part of an authentication protocol. Typically, an authentication protocol would run before the two communicating parties run some other protocol (e.g., a reliable data transfer protocol, a routing table exchange protocol, or an email protocol). The authentication protocol first establishes the identities of the parties to each others' satisfaction; only after authentication do the parties get down to the work at hand.
As in the case of our development of a reliable data transfer protocol,
in Chapter 3, we will find it instructive here to develop various versions
of an authentication protocol, which we will call ap ("authentication
protocol"), and poke holes (i.e., find security flaws) in each version
as we proceed. Let's begin by assuming that Alice needs to authenticate
herself to Bob.
Figure 7.3-1: Protocol ap1.0 and a failure scenario.
Figure 7.3-2: Protocol ap2.0 and a failure scenario.
Given that we have now studied both the network and data link layers,
we know that it is not that hard (e.g., if one had access to the operating
system code and could build one's own operating system kernel, as is the
case with Linux and several other freely available operating systems)
to create an IP datagram, put whatever IP source address we want
(e.g., including Alice's well-known IP address) into the IP datagram and
send the datagram over the link layer protocol to the first hop router.
From then on, the incorrectly-source-addressed datagram would be dutifully
forwarded to Bob. This approach is a form of IP spoofing, a well-known
security attack technique [Cert 96].
IP spoofing can be avoided if a router is configured to refuse IP datagrams
that do not have a given source address. For example, Trudy's first
hop router could be configured to only forward datagrams containing Trudy's
IP source address. However, this capability is not universally deployed
or enforced. Bob would thus be foolish to assume that Trudy's network manager
(who might be Trudy herself!) had configured Trudy's first hop router
to only forward appropriately-addressed datagrams.
Figure 7.3-3: Protocol ap3.0 and a failure scenario.
The security flaw here is clear. If Trudy eavesdrops on Alice's
communication, then she can learn Alice's password. Lest you think
this is unlikely, consider the fact that when one Telnet's to another machine
and logs in, the login password is sent unencrypted to the Telnet server
. Someone connected to the Telnet client or server's LAN can
possibly "sniff" (read and store) all packets transmitted on the LAN and
thus steal the login password. In fact, this is a well-known approach
for stealing passwords (see, e.g., [Jimenez 1997].
Such a threat is obviously very real, so ap3.0 clearly won't do.
While it is true that ap3.1 prevents Trudy from learning Alice's password, the use of cryptography here does not solve the authentication problem! Bob is again subject to a so-called playback attack: Trudy needs only eavesdrop on Alice's communication, record the encrypted version of the password, and then later play back the encrypted version of the password to Bob to pretend that she is Alice. The use of an encrypted password doesn't make the situation manifestly different from that in Figure 7.3-3.
Rather than just stop here with this solution, however, let us consider a more general approach for combating the playback attack. The failure scenario in Figure 7.3-3 resulted from the fact that Bob could not distinguish between the original authentication of Alice and the later playback of Alice's original authentication. That is, Bob could not tell if Alice was "live" (i.e., was currently really on the other end of the connection) or whether the messages he was receiving were a recorded playback of a previous authentication of Alice. The very (very!) observant reader will recall that the 3-way TCP handshake protocol needed to address the same problem - the server side of a TCP connection did not want to accept a connection if the received SYN segment was an old copy (retransmission) of a SYN segment from an earlier connection. How did the TCP server side solve the problem of determining if the client was really "live"? It chose an initial sequence number (which had not been used in a very long time), sent that number to the client, and then waited for the client to respond back with an ACK segment containing that number. We can adopt the same idea here for authentication purposes.
A nonce is a number that a protocol will only ever use once-in-a-lifetime. That is, once a protocol uses a nonce, it will never use that number again. Our ap4.0 protocol uses a nonce as follows:
Figure 7.3-4: Protocol ap 4.0: no failure scenario.
The operation of protocol ap5.0 is illustrated in Figure 7.3-5.
Is protocol ap5.0 as secure as protocol ap4.0? Both use nonces. Since ap5.0 uses public key techniques, it requires that Bob retrieve Alice's public key. This leads to an interesting scenario, shown in Figure 7.3-6, in which Trudy may be able to impersonate Alice to Bob:
In the scenario in Figure 7.3-6, Bob and Alice might together eventually discover that something is amiss, as Bob will claim to have interacted with Alice, but Alice knows that she has never interacted with Bob. There is an even more insidious attack that would avoid this detection. In the scenario in Figure 7.3-7, both Alice and Bob are talking to each other, but by exploiting the same hole in the authentication protocol, Trudy is able to transparently interpose herself between Alice and Bob. In particular, if Bob begins sending encrypted data to Alice using the encryption key he receives from Trudy, Trudy can recover the plaintext of the communication from Bob to Alice. At the same time, Trudy can forward Bob's data to Alice (after re-encrypting data using Alice's real public key).
Bob is happy to be sending encrypted data, and Alice is happy to be
receiving data encrypted using her own public key; both are unaware of
Trudy's presence. Should Bob and Alice meet later and discuss their
interaction, Alice will have received exactly what Bob sent, so nothing
will be detected as being amiss. This is one example of the so-called
man-in-the-middle attack (more appropriately here, a "woman-in-the-middle"
attack). It is also sometimes known as a bucket-brigade attack,
since Trudy's passing of data between Alice and Bob resembles the passing
of buckets of water along a chain of people (a so-called "bucket
brigade") who are putting out a fire using a remote source of water.
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Copyright Keith W. Ross and James F. Kurose 1996-1999 . All rights reserved.